Handle NULL values ​in trigger of PostgreSQL views using DEFAULT?

I wanted to have an explanation on triggers of Postgres views.

To make clear what I want to ask, I'll give you a very simplified example of my case. In this example we have two tables (table_a, table_b) that joined together make the view in the example (vw_table_ab).

In this example I will use trivial names and simple DDLs/DMLs.

-- TABLE table_a
CREATE TABLE table_a
(
    id              serial PRIMARY KEY,
    timestamp_field timestamp DEFAULT now() NOT NULL,
    boolean_field   boolean   DEFAULT FALSE NOT NULL
);

-- TABLE table_b
CREATE TABLE table_b
(
    id              serial PRIMARY KEY,
    timestamp_field timestamp DEFAULT now() NOT NULL,
    boolean_field   boolean   DEFAULT FALSE NOT NULL,
    id_table_a      integer                 NOT NULL,
    CONSTRAINT "fk_table_a" FOREIGN KEY (id_table_a) REFERENCES table_a (id) ON DELETE CASCADE NOT DEFERRABLE,
    CONSTRAINT "u_table_a" UNIQUE (id_table_a)
);

-- VIEW vw_table_ab
CREATE VIEW vw_table_ab AS (
    SELECT a.timestamp_field AS timestamp_a,
           a.boolean_field   AS boolean_a,
           b.timestamp_field AS timestamp_b,
           b.boolean_field   AS boolean_b
    FROM table_a a
    JOIN table_b b ON a.id = b.id_table_a
);

A trigger function on standard actions (INSERT, UPDATE and DELETE) is linked to this view through an INSTEAD OF trigger.

-- TRIGGER FUNCTION fn_trigger
CREATE FUNCTION fn_trigger() RETURNS trigger LANGUAGE plpgsql AS
$_$
DECLARE
    sql TEXT;
BEGIN
    sql = 'SELECT ' || TG_TABLE_NAME || '_' || lower(TG_OP) || '($1);';

    IF TG_OP = 'INSERT' OR TG_OP = 'UPDATE' THEN
        EXECUTE (sql) USING NEW;
        RAISE NOTICE '%', sql;
        RETURN NEW;
    ELSE
        EXECUTE (sql) USING OLD;
        RAISE NOTICE '%', sql;
        RETURN OLD;
    END IF;
END;
$_$;

-- TRIGGER tr_table_ab
CREATE TRIGGER tr_table_ab
INSTEAD OF INSERT OR UPDATE OR DELETE ON vw_table_ab
FOR EACH ROW EXECUTE PROCEDURE fn_trigger();

The example I bring has a trigger called only on the insert action, and the function that is executed is this:

-- INSERT FUNCTION vw_table_ab_insert
CREATE FUNCTION vw_table_ab_insert(new vw_table_ab) RETURNS void LANGUAGE plpgsql AS
$_$
DECLARE
    id_table_a integer;
BEGIN
    INSERT INTO table_a (timestamp_field, boolean_field) VALUES (new.timestamp_a, new.boolean_a) 
    RETURNING id 
    INTO id_table_a;

    INSERT INTO table_b (timestamp_field, boolean_field, id_table_a) VALUES (new.timestamp_a, new.boolean_b, id_table_a);
END;
$_$;

Now we can get to my problem. I make an insert on the view, and when the action is triggered, I get a "Not null violation" error becouse I have some NOT NULL constraints on table_a and table_b like in this case:

INSERT INTO vw_table_ab (timestamp_a, boolean_a, timestamp_b, boolean_b) VALUES (now(), NULL, now(), NULL);

Suppose that the previous statement is generated through a programming language framework and I don't want to handle this case in backend code, but I want handle this case in PostgreSQL in the insert function vw_table_ab_insert. So at this point my problem is bound to the new parameter of the function because I have fields of the view that are NULL. But these fields have a DEFAULT value in the definition of the base table, and I want to use that.

...
timestamp_field timestamp DEFAULT now() NOT NULL,
boolean_field   boolean   DEFAULT FALSE NOT NULL
...

My question: How can I manage the NULL values ​​in trigger of the views using the DEFAULT defined in the tables?

Initially I thought of putting IF ... THEN ... inside the function and override null values ​​with DEFAULT expression but I do not really like that. For example, the function would become like this:

CREATE FUNCTION vw_table_ab_insert(new vw_table_ab) RETURNS void LANGUAGE plpgsql AS
$_$
DECLARE
    id_table_a integer;
BEGIN
    IF new.timestamp_a IS NULL THEN
        new.timestamp_a = DEFAULT;
    END IF;

    IF new.boolean_a IS NULL THEN
        new.boolean_a = DEFAULT;
    END IF;

    IF new.timestamp_b IS NULL THEN
        new.timestamp_b = DEFAULT;
    END IF;

    IF new.boolean_b IS NULL THEN
        new.boolean_b = DEFAULT;
    END IF;

    INSERT INTO table_a (timestamp_field, boolean_field)
    VALUES (new.timestamp_a, new.boolean_a) 
    RETURNING id
    INTO id_table_a;

    INSERT INTO table_b (timestamp_field, boolean_field, id_table_a)
    VALUES (new.timestamp_a, new.boolean_b, id_table_a);
END;
$_$;

Someone can help me? Is there another method for handling this case?